Distortion is a significant problem in telecommunication systems. While there are several types, distortion generally can be thought of as some modification of a waveform by introducing features not present in the original waveform or by suppressing or modifying features present in the original waveform. The present invention is particularly concerned with intermodulation distortion. Intermodulation is the modulation of components of a complex wave by each other producing waves having frequencies, among others, equal to the sums and differences of those components of the original wave. In other words, intermodulation distortion results from spurious combination-frequency components in the output of a transmission system.
One environment where intermodulation distortion is a problem is in radio base stations used in mobile telecommunications systems. Such base stations typically include plural transmitters and plural receivers, (i.e., plural transceivers), and may employ a duplexer so that transmitter-receiver pairs can share an antenna for both transmitting and receiving radio frequency signals. For example, in the well-known GSM 900 mobile communications system, the transmit band covers 35 MHz and includes a 175 transmit frequencies approximately 200 KHz wide. Unfortunately, transmissions over certain pairs of these 175 transmit frequencies generate significant intermodulation products in the receive band. As a result, frequency planning methods are required in order to avoid using such frequency pairs. Another problem is that unused frequency pairs reduce the capacity of the base station.
These problems are overcome by compensating for intermodulation distortion in a received signal caused by transmission of first and second signals on first and second transmit radio frequencies. In particular, after detecting a received signal, the intermodulation distortion is suppressed, reduced, and/or eliminated using information that is associated with the first and second transmitted signals. The information associated with the first and second transmitted signals is used to calculate or otherwise obtain a correction factor that corresponds to the intermodulation distortion. The correction factor may be subtracted or otherwise removed from the received signal to suppress the intermodulation distortion.
In a preferred, non-limiting, example embodiment, a compensator in the receiver detects (1) the baseband information corresponding to the first and second transmitted signals, (2) the power level at which the first and second signals were transmitted, (3) the frequencies over which the first and second signals were transmitted, and (4) timing information associated with the transmission of those signals. The baseband information, the power level, and the frequency corresponding to each of the signals are used to calculate or retrieve from a lookup table a compensation factor corresponding to the intermodulation distortion. The timing information is used to synchronize that calculation or table lookup for a particular signal. One or more time delays is used to synchronize when the compensation signal is removed from the received signal. Such delays may take into account a first delay associated with a time period for the first signal to be processed and transmitted via an antenna. A second delay may be associated with a time period for the second signal to be processed and transmitted via the antenna. A third delay may be associated with a time period for receiving a signal via the antenna and processing it through the compensator.
There are a number of benefits of the present invention. For example, it may be implemented without changing existing radio hardware. Parameters in the lookup table may be updated adaptively to account for changes due to temperature, aging, and different behaviors of different frequency pairs. Such a flexible software solution is further advantageous because compensation can be made on an as needed basis adaptively tailored to individual sites and/or base stations. Another potential benefit is that IM distortion requirements on hardware units may be relaxed, which allows for less expensive hardware. Moreover, frequency planning, which otherwise would be required when using a duplexer to avoid intermodulation frequency pairs, is not necessary. As a result, the capacity of a transceiver using a duplexer is not reduced as a result of not using those frequency combinations that result in intermodulation distortion.